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Author Topic: Solution for the 5V vs 3.3V problem  (Read 10379 times)
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Hey Rob,

Here is what I'm thinking. This first drawing is a rough schematic for just the crosspoint section of pins. Other pins could be added to these and all the rest of the pins would need to be passed through 74LVC16245's as well.



Red trace is on bottom

Blue is on top

Black is on bottom (5v, 3.3v, gnd)

To answer you question about the pins, Mayhew Labs came up with a pretty nice solution.

http://dl.dropbox.com/u/55783756/Photos/go_between_shield_front%20(1).jpg

http://dl.dropbox.com/u/55783756/Photos/go_between_shield_back%20(1).jpg

As you can see, the headers don't go through the pcb, but are instead surface mounted. This way, we could have separate top and bottom circuits.


Dan


* possible diagram.png (24.91 KB, 962x492 - viewed 26 times.)
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Here's the schematic & layout, please create a version with the parts you are suggesting.
Top & bottom mount surface mount headers would be one way to keep the physical board the same size, as long as its cut back from the USB header.

* DUE_adapter_shield.brd (268.82 KB - downloaded 27 times.)
* DUE_adapter_shield.sch (228.03 KB - downloaded 25 times.)
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@OsuDan
At this stage a simple block diagram would be clearer, too many parallel lines for me to follow there.

_____
Rob
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This is one way I see it working,



It would need the above for every 8 pins you wanted to handle, but if you stick to the standard shield (not Mega) that's only 3x the above I think.

Bear in mind that some control circuitry would be required as well.

XPS = crosspoint switch
BUF = buffer, probably TXB0108

______
Rob
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Also take a look at our http://playground.arduino.cc/GorillaBuilderz/LeoShield which is another pin adaptor.

Dale
www.GorillaBuilderz.com

Hey guys,

I've been looking into something like this for a while as well. One thing that caught my eye was this pin converter shield for the UNO/Leo to go between the old pinout and 1.0

http://mayhewlabs.com/products/go-between-shield

Something like this could be implemented with logic converters between the connection points. This could also leads in some other interesting directions as well:
  • Being able to select/designate individual pins as 5v or 3.3v logic
  • Integrating dynamic pin assignment so the pinout could be adaptable to older shields configurations
  • And if you really wanted to get crazy, you could even wire the whole thing up to be driven by an integrated IC so you just add a library to your sketch and assign the pinout and voltage in software.
  • And since I've already gone way out in left field, to complete the prefect conversion shield, add either an IC to linearly scale the analog inputs or a separate 5v 12 or 16 bit ADC

EDIT: Just read the post about the GTL2000 and it got me thinking there has got to be a chip that could route pins and shift logic levels... Yup.

http://www.intersil.com/en/products/audiovideo/video-switching/unbuffered-crosspoint-switches/CD22M3494.html

Thoughts?


* leoshield-perspective-320x320.jpg (22.17 KB, 320x320 - viewed 57 times.)
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Ok so here's the diagram of the circuit as I see it. Let me know what you guys think

Circuit for pins who's position might need to be changed



And here is for the other pins who's position is fixed



For both the CD22M3494 (crosspoint) and the SN74LVC16T245 (logic shifter) I think the buses can be 16 bit wide. As for going with the SN74LVC16T245 over the TXB0108, the advantage I see is that with the SN74LVC16T245, you can select each pin's output so some pins could interface with 5v logic, while others with devices at 3.3v. Might as well keep the shield dynamic as we can, especially if things really are moving towards 3.3v logic. Just my two cents, what's your guys reaction?


Dan
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the advantage I see is that with the SN74LVC16T245, you can select each pin's output
I can't see where the data sheet says this is possible, apart from two OE and two DIR pins there are no control signals.

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Rob
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Here's the schematic & layout, please create a version with the parts you are suggesting.
Top & bottom mount surface mount headers would be one way to keep the physical board the same size, as long as its cut back from the USB header.
I tried the TXB0108 and finally gave up.
The blue boxes in the attached picture contain 3 pcs pf TXB0108.
I made the beta board and tested it.
While I test the TXB0108 5V output with voltmeter, the voltage is 3V.
However, I test it with oscilloscope, the output is 5V. I touch the wire or chip with my finger, the voltage became unstable.
Well, I think, the TXB0108 is easy to go into self-excited oscillation process. And the load capacity is small. If I connect other devices to it through long cables, such as a digital sensor, the output may have problem. However, if I intert the sensor in the header of the beta DUE board directly, it works fine.
I tried many time but still didn't success.
Not sure if the TXB0108 performs not good enough.


* 1st version-1.jpg (123.61 KB, 720x381 - viewed 79 times.)
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I can't see where the data sheet says this is possible, apart from two OE and two DIR pins there are no control signals.

______
Rob

Hey Rob,

Guess I had an optimistic read on that datasheet, you are right. Here is my solution.



With the setup, each pin can be selected as 3.3v or 5v through the MUX, then routed through the logic shifter if it is 5v. After that, the crosspoint handles the pin reassignment if needed. How about this? We getting close?

Dan
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I assume that all the data busses are 8 bits. I don't get how the Logic shifter has bi-directional signals. What's on the 3v3 side?

TBH I'm treating this as an academic exercise because it's interesting, personally I really can't see much use for such a complex shield but I'm happy to help with a design if you do.

_____
Rob
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I have not looked at the circuits to evaluate....but i want one of these!
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i want one of these!
Maybe there is a market after all, I always was 180 degrees out of phase with the consumer smiley

Dan, my last question? How does the logic shifting circuit have bi-directional IO?

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Rob
« Last Edit: December 20, 2012, 02:08:38 am by Graynomad » Logged

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Maybe there is a market after all, I always was 180 degrees out of phase with the consumer smiley

Dan, my last question? How does the logic shifting circuit have bi-directional IO?
_____
Rob

I would say if there is a market for the Due - then there is a market for this shield.  To choose the Due over the raspberry pi the main feature is the general purpose IO and the existing shields.  But to use either of these such a board is extremely useful.

Not only would a significant percentage of Due purchasers want such a shield- there would also be a market of people who current do NOT choose a Due to to shield incompatibility!


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I just thought I'd resurrect this thread because I now have a good reason to actually do something like this.

Did anyone take this any further?

Going back to something I missed in the original post by giantsfan3

Quote
the above level-translator IC based solution should work bi-directionally, for all signal types (digital I/O, PWM, analog lines, Serial, SPI, I2C),

Does the GTL2000 pass analog OK, it's only a few FETs so I would assume so but haven't tried it.

______
Rob

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I posted the question of how I can convert my 5 volt output from the hall effect sensors to a 3.3 volt output that the due can read.  I posted this in another forum.

I've been looking at these low voltage translators and I don't think I can use these. 

I want the 3.3 volts to change linearly as the 5 volt changes...ie when I am getting 2.5 volts from the sensor the arduino should be reading 1.65 volts.

I'm not sure if I should just use a voltage divider because I'm concerned with accuracy to a certain degree.

Could you please help me out...this is for my senior design project.
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